In a conventional DC-DC converter, a transformer may be used for the purpose of isolating a drive battery. This type of DC-DC converter is known as an isolated DC-DC converter. Further, leakage inductance exists in an isolation transformer.
Here, when the leakage inductance of the isolation transformer is large, the following problems occur. Firstly, eddy current loss increases. Secondly, LC resonance occurs due to respective electrostatic capacity components of the leakage inductance and the isolation transformer and a circuit connected to the isolation transformer, resulting in an increase in high frequency noise.
As a method for solving the problems described above, respective distances between a primary side coil and two secondary side coils are uniformly minimized by winding the primary side coil on a single plane rather than dividing the primary side coil, and disposing the primary side coil between layers formed by the two secondary side coils (see PTL 1, for example). With this method, the leakage inductance is reduced.